Remove all items possible from the restroom. This includes items from the counter, towel bars and the covers from light switches and electrical sockets. These last items can be unscrewed from their housing.
Cover the floor, counter tops and bathtub with plastic tarp, taped in place with low-tack painter's tape. Wrap the toilet in more tarp and tape it in place.
Line all areas that you do not want painted with painter's tape. This may include mirrors, windows and ceiling trim.
Fill in any holes and cracks with drywall compound, scraped smooth with a putty knife. Allow the compound to dry completely. Many brands change colors as they dry, making it easy to tell when it's ready.
Remove any flaking paint with a paint scraper.
Sand old gloss paint with 220-grit sandpaper and a sanding block until the shine is gone. This will allow the new paint to adhere more securely. Wipe the walls with a damp rag to remove sanding dust.
Add primer into a paint tray or a bucket fitted with a paint screen.
Roll a 4-inch paint roller into the primer, then roll off the excess on the paint tray's upper portion or the paint screen within the bucket.
Apply a coat of primer to the walls, beginning along the edges with the 4-inch paint roller. Once the edges are complete, switch to a full-sized roller and apply the primer in overlapping vertical zig-zag strokes.
Allow the primer to dry for the time recommended on the can.
Lightly sand the primer with 220-grit sandpaper to prepare the surface for paint. Wipe the surface clean with a damp rag.
Apply a coat of gloss paint, following the same steps you used to apply the primer. Use a paint designed for a bathroom if possible.
Allow the paint to dry, then apply a second coat if desired. Allow that coat to dry for 24 hours.
Score the edge of the painter's tape with a razor blade before removing it. This prevents any paint on the tape from pulling paint off of the wall.
Remove the tape and tarps, and replace the items in the restroom.